1. Field of the Invention
The present invention relates to an electrophotographic type image forming apparatus such as a copying machine or printer which can form a multi-color image.
2. Description of the Related Art
As an image forming apparatus such as a copying machine or page printer, an electrophotographic type image forming apparatus has been known conventionally. In the electrophotographic type image forming apparatus, the surface of a drum-like photosensitive member (to be referred to as a “photosensitive drum” hereinafter) serving as an image bearing member is uniformly charged and exposed by an exposure device based on image information, thereby forming an electrostatic latent image. A developing device develops the electrostatic latent image formed on the photosensitive drum as a toner image. The toner image is formed onto a transfer medium such as paper by using electrostatic force. Then, the toner image is fused and fixed onto the transfer medium by heat and pressure applied by a fixing device, and is discharged as an output image. Recent electrophotographic type image forming apparatuses are achieving advanced functions such as color printing and higher speed. Color image forming apparatuses for an electrophotographic process, which include an intermediate transfer belt for coping with these functions, have been widely employed.
As an example of electrophotographic type color image forming apparatuses, there is known an image forming apparatus which adopts an intermediate transfer belt as an intermediate transfer member. Toner images in yellow, magenta, cyan, and black (to be referred to as Y color, M color, C color, and Bk color hereinafter) are formed by a plurality of image forming stations and transferred to the intermediate transfer belt. These toner images are then transferred at once to a recording medium at a secondary transfer portion.
Japanese Patent Application Laid-Open No. 2010-102320 discloses an arrangement in which the primary transfer roller of each image forming station is moved to bring an intermediate transfer belt and each photosensitive drum into contact with each other or separate them from each other.
However, the intermediate transfer belt generates a flutter or ruffle to a certain degree during rotation. To reliably separate the intermediate transfer belt and primary transfer roller in consideration of these factors, a separation amount as large as possible is required. This sometimes hinders downsizing of the image forming apparatus. This problem arises not only in the intermediate transfer belt but also in an arrangement which adopts a conveyance belt for conveying a transfer medium.